1. Field of the Invention
The present invention relates to a technique, for use in a motorcycle that controls a belt-type continuously variable transmission by hydraulic pressure to discharge hydraulic oil containing air that is immixed therein due to inclining of the motorcycle in the left-right direction.
2. Description of the Related Art
Conventionally, there are available belt-type continuously variable transmissions that are controlled by hydraulic pressure (e.g., Japanese Patent Laid-open Publication No. Hei11-82725). In such a continuously variable transmission, an oil chamber is arranged in each of paired pulleys. Each of the pulleys sandwiches the belt by hydraulic pressure. An oil passage is connected to the oil chamber of each pulley, and is provided with a control valve that is controlled based on a driving state of the vehicle, such as, e.g., a throttle opening degree and a vehicle speed. Some of such control valves open and close a discharge port for discharging the hydraulic oil in the oil passage so that the hydraulic pressure becomes equal to a pressure according to a signal from the controller. When a pulley needs a large clamping force (a force for sandwiching the belt), such as when, e.g., a large torque is input to the continuously variable transmission, the control valve reduces the discharge amount of hydraulic oil by closing the discharging port, to thereby increase the hydraulic pressure in the oil passage.
However, a motorcycle equipped with such a hydraulic pressure-controlled continuously variable transmission has a problem described below. The motorcycle may be largely inclined in the left-right direction, and in such a case, the surface of the hydraulic oil in the oil sump will vary in its height such that the inlet of the oil pump emerges out of the hydraulic oil. Consequently, air may be sucked by the oil pump together with the hydraulic oil to be immixed into the hydraulic oil in the oil passage. This makes it difficult to produce a sufficient hydraulic pressure.
In connection with this problem, when the discharge port is open after the motorcycle returns to its original upright posture, the air-mixed hydraulic oil is gradually discharged from the oil passage through the discharge port while the oil pump newly supplies hydraulic oil into the oil passage. However, when the above described control based on a driving state of the motorcycle is carried out after the motorcycle returns to its normal, upright posture and the discharge port is thus closed, the hydraulic oil cannot be readily discharged from the oil passage. Consequently, the air-mixed hydraulic oil will stay in the oil passage for a longer period of time.